1. Field of the Invention
The present invention relates to a pallet transfer system for transferring a pallet, on which a work piece is placed, between a machine tool, e.g., a horizontal machining center, and a pallet conveying device having a stacker crane.
2. Description of the Background Art
Patent document 1 discloses a conventional machine tool having a pallet exchanger where a pallet on which a work piece is placed is exchanged between a machining-side position and a stand-by position by rotation of the pallet. In the conventional machine tool as disclosed in Patent document 1, a stacker crane is used when a pallet is exchanged and carried between the stand-by position of the machine tool and a pallet housing station which is separately provided from the machine tool. The stacker crane is capable of moving on a rail laid between the machine tool and the pallet housing station, being advanced and retreated by arms, and being raised and lowered. In the conventional machine tool, the stacker crane is moved to a predetermined position on the rail, being advanced toward the stand-by position by extension of the arms, being raised, and receiving a pallet from the stand-by position, and carrying the received pallet to the pallet housing station after retreating the pallet toward a rail side by retracting of the arms. It should be noted that the transferring operation where the pallet is transferred from the stacker crane to a stand by position is made in reverse order to the receiving operation of the pallet. The stacker crane is advanced toward the stand-by position by extension of the arms at the predetermined position, and then lowered so as to transfer a pallet to the stand-by position.
However, when a pallet is directly received and transferred between the stand-by position and the stacker crane as described above, the extending distance of arms in the stacker crane may be long. Therefore, the weight of a work piece placed on a pallet is limited depending on strength of the arms. Further, when the size of arms (that is, a stacker crane) is increased in order to improve the strength of the arms, such a crane cannot be applied to a conventional machine tool, and a larger installation space and operation space become necessary, which results in high cost.
Therefore, for example, Patent document 2 discloses a conventional machine tool in which an intermediate station is provided between a predetermined position of a stacker crane and a stand-by position so as to shorten the extending distance of arms.
[Patent document 1] Japanese Unexamined Patent Publication No. 2003-340673
[Patent document 2] Japanese Unexamined Patent Publication No. 1986-33416
However, when an intermediate station is provided, the device size is enlarged and the structure of the device becomes complicated. Therefore, smooth transfer of a pallet between a stand-by position of a machine tool and a stacker crane and rationalization of the structure of the transfer are needed.
Therefore, an object of the present invention is to provide a pallet transfer system capable of smoothly transferring a pallet between a machine tool and a pallet conveying device, and rationalize the structure.